If checkbox field is checked, change multiple choice option to something specific. Form rule?

This is the current form rule that I have created, the only problem is that it only forces the change in the "Edit" mode, so I would have to click "Edit' and click "Save" every time I want to change it.

Is there any way to fire this change without having to click "Edit" -> "Save"? I also do not want to change the [Multiple Choice] field to a formula rule.

Maybe script to recognize if [A] is checked, then EditRecord for [Multiple Choice]?

Yes, a script could be written to do that, or possibly another technique, but why would you want that. I would suggest a multiple choice field like you have now, for updates, but then a formula field to calculate the final choice based on a formula.If you use a formula it will always be the correct result, where as a script solution would only be correct right after the script was run.